a) Ta có:
` +) x/35 = -6/9`
=> `x = -6/9. 35`
=> `x= -70/3`
`+) 18/z = -3/7`
=> `z= 18 : (-3/7)`
=> `z= 18. (-7/3)`
=> `z= -42`
Vậy `x= -70/3` ; `z= -42`
b) `(x+5)/-25 = -4/(x+5)`
=> `(x+5)(x+5) = -4.(-25)`
=> `(x+5)^2 = 100`
=> \(\left[ \begin{array}{l}(x+5)^2=10^2\\(x+5)^2=(-10)^2\end{array} \right.\)
=> \(\left[ \begin{array}{l}x+5=10\\x+5=-10\end{array} \right.\)
=> \(\left[ \begin{array}{l}x= 5\\x=-15\end{array} \right.\)
Vậy `x= 5` hoặc `x= -15`
